Congenital Pelger-Huët anomaly in a horse

A 1.5-year-old male Arabian horse was referred to the Louisiana State University Veterinary Teaching Hospital and Clinic for an open deep laceration involving two thirds of the right trunk. The initial CBC results included an inflammatory leukogram, characterized by a marked degenerative left shift consisting of only immature band neutrophils (7500/microL, reference interval 0-100/microL) with toxic changes and no segmented neutrophils (0/microL, reference interval 2700-6700/microL). On abdominal ultrasonography, free abdominal fluid was found and collected for analysis. Abdominal fluid had a marked increase in total nucleated cells (40,600 cells/microL) consisting of 74% nondegenerate neutrophils that all were hyposegmented, with mature condensed chromatin. Re-evaluation of neutrophil morphology on the initial blood smear confirmed hyposegmentation and mature condensed chromatin, similar to that observed in cells in the abdominal fluid. A diagnosis of Pelger-Huët anomaly (PHA) was made in this colt. Congenital PHA was documented on the basis of persistent neutrophil hyposegmentation on serial blood smears, ruling out of acquired causes of PHA, and findings of similar neutrophil hyposegmentation on blood smears from the colt's sire and the sire's siblings. To our knowledge, this is the first reported case of congenital PHA in a horse.